Curable coating compositions such as thermoset coatings are widely used in the coatings art. They are often used for topcoats in the automotive and industrial coatings industry. Color-plus-clear composite coatings are particularly useful as topcoats where exceptional gloss, depth of color, distinctness of image, or special metallic effects are desired. The automotive industry has made extensive use of these coatings for automotive body panels. Color-plus-clear composite coatings, however, require an extremely high degree of clarity in the clearcoat to achieve the desired visual effect.
High-gloss coatings also require a low degree of visual aberrations at the surface of the coating in order to achieve the desired visual effect such as high distinctness of image (DOI).
Such coatings are especially susceptible to a phenomenon known as environmental etch. Environmental etch manifests itself as spots or marks on or in the finish of the coating that often cannot be easily rubbed out.
It is also often desirable to provide options of different types of carbamate-functional materials to provide coatings with a good combination of properties such as durability, hardness, flexibility, and resistance to scratching, marring, solvents, and acids.
Curable coating compositions based on curable components having carbamate functionality have been described in the art to provide etch-resistant coatings, e.g., U.S. Pat. No. 5,356,669 and WO 94/10211. Non-polymeric carbamate-functional compounds for coating compositions have been described in U.S. Pat. No. 5,336,566 and EP 636,660.
In order to obtain the smooth finishes that are often highly desirable in the coatings industry, coating compositions preferably tend to be fluid in nature, and to exhibit good flow. Good flow is observed when the coating composition is fluid enough at some point after it is applied to the substrate and before it cures to a hard film so that the surface of the coating takes on a smooth appearance. Some coating compositions exhibit good flow immediately upon application and others exhibit good flow when heated. One way to impart fluid characteristics and good flow to a coating composition is to incorporate volatile organic solvents into the compositions. These solvents can provide the desired fluidity and flow during the coating process, after which they evaporate, leaving only the coating components behind. However, the use of such solvents also increases the volatile organic content (VOC) of the coating composition. Because of the adverse impact VOC has on the environment, many government regulations impose limitations on the amount of solvent that can be used. It would thus be desirable to utilize coating composition components that provide good fluidity and flow to the coating composition without the need for large amounts of solvent.
Because of their other beneficial properties, it would also be desirable to provide carbamate-functional compounds for use in coating compositions that do not require large quantities of solvent.
It is often desirable that such coating compositions incorporate one or more pigments. Pigments may be used to provide a variety of appearance and/or performance characteristics. For example, it is often advantageous that a coating composition provide a sparkle, pearlescent, luminescent, and/or metallic appearance or an increased depth of color. Such characteristics and/or appearances can often be achieved with the use of one or more effect pigments as defined below.
The quality of the appearance achieved with the use of one or more effect pigments is often dependent upon the orientation of the individual effect pigment particles. For example, it is generally believed that optimal appearance of a metallic pigment such as aluminum is obtained when the individual aluminum flakes are positioned to lay parallel or flat to the coating surface.
Improved or desirable orientation of effect pigments in a cured coating composition may be manifested as the degree of change in lightness values as a function of the viewing and/or illuminating geometry. Users of coatings containing one or more effect pigments, especially metallic coatings, typically require a degree of change as large as possible. This is particularly so for metallic coating compositions intended for use in the automotive market. Such coatings are said to have good xe2x80x9ctravelxe2x80x9d, that is, the perceived lightness of a cured coated panel viewed at a first angle is significantly different from the perceived lightness of that same panel viewed at a second angle some distance from the first angle. This may also be characterized as flop per ASTM E-284-96. In either case, a significant difference in appearance between the two viewing angles is desireable.
Good travel is generally believed to be observed when the effect pigment particles have a uniform parallel orientation to the surface of the coating or substrate.
Control of effect pigment orientation has long presented a challenge to those skilled in the art. This is particularly true for topcoat technologies utilizing basecoat/clearcoat and/or wet on wet applications. Effect pigment particle orientation in a basecoat is often susceptible to disordering by the application of a clearcoat, especially if a wet on wet application is employed. Destabilization of pigments during the manufacture of a curable coating composition can also result in negative appearance and performance properties such as loss of hiding and decreased metallic effect if the effect pigment is a metallic pigment. Disordering and/or destabilization often manifest as floculation and/or agglomeration of the effect pigment. Effect pigment orientation, especially that of metallic pigments such as aluminum, has been particularly difficult to control in solvent borne coatings, especially high solids solvent borne coatings.
Thus, it would be desirable to provide curable coating compositions possessing the foregoing advantages as well as improved control over the orientation and/or appearance of effect pigments, especially metallic pigments. Such compositions would be particularly useful as topcoats alone or in conjunction with a clearcoat.
According to the present invention, there is provided a curable coating composition having improved effect pigment orientation, the composition comprising:
(A) a carbamate-functional component having a hydrophobicity equivalent to or greater than that of a carbamate-functional compound (Axe2x80x2) which is the reaction product of:
(1) a compound comprising a plurality of hydroxyl groups, at least one of which is the result of a ring-opening reaction between an epoxy group and an organic acid group, and
(2) cyanic acid or a compound comprising a carbamate group, and
(B) a component comprising a plurality of groups that are reactive with the carbamate functional groups on component (A), and
(C) at least one effect pigment, the curable coating composition when cured having a substantially improved effect pigment orientation. Preferably, the compositions of the invention also comprise one or more rheology control agents such as microgels, especially nonaqueous dispersions of microgels.
The invention also provides methods for obtaining coated surfaces having improved effect pigment orientation and appearance, comprising
providing at least one effect pigment,
providing a carbamate-functional component (A) having a hydrophobicity equivalent to or greater than that of a carbamate-functional compound (Axe2x80x2)which is the reaction product of:
(1) a compound comprising a plurality of hydroxyl groups, at least one of which is the result of a ring-opening reaction between an epoxy group of a compound (a) and an organic acid group of a. compound (b), and
(2) cyanic acid or a compound comprising a carbamate group,
providing a component (B) comprising a plurality of groups that are reactive with the carbamate functional groups on component (A)
mixing components (A), (B) and (C) so as to provide a curable coating composition (I),
coating a surface with the curable coating composition (I), and
exposing the coated surface to heat for a time sufficient to cure the coating composition (I) to provide a cured coated surface having improved effect pigment orientation.
The method and curable coatings compositions of the invention provide measurable and unexpected improvement with respect to the orientation of at least one effect pigment. In particular, the substantially improved effect pigment orientation is measured as a change in lightness value as a function of viewing and/or illumination geometry. As a result, compared to the prior art, cured coatings according to the invention illustrate an unexpectedly large degree of change in their lightness values as a function of the viewing and/or illuminating geometry. Cured coatings according to the instant invention having at least one effect pigment (C) will generally exhibit an MF-D greater than or equal to 60 and preferably greater than 70, as measured on a multi-geometry spectrophotometer capable of measuring a lightness value at 25xc2x0 off specular (L@25xc2x0) and a lightness value at 70xc2x0 off specular (L@70xc2x0), with a D65 illuminant source and a 10 degree standard observer per ASTM #-308-96, wherein ME-D=[((L@25xc2x0)xe2x88x92(L@70xc2x0))/(L@70xc2x0)]xc3x9750.
Compositions of the present invention can also reduce the need for organic solvents, and can also impart to coating compositions the ability to spray apply at high viscosities while still maintaining good flow and appearance characteristics.
The present invention provides coatings having a good combination of properties such as durability, hardness, and resistance to scratching, marring, solvents, and acids. Coating compositions according to the invention can also provide low VOC levels while maintaining other beneficial properties that are often found in coating compositions containing relatively high amounts of solvent, such as good sag resistance, leveling, low orange peel, gloss, DOI, wetting of the substrate, and pigment dispersing and loading, and uniform cure.